Polymers and oligomers having carbamate functional groups have been used in a variety of curable compositions. Carbamate-functional acrylic polymers are described, for example, in U.S. Pat. No. 5,356,669 and WO 94/10211. These can be prepared by addition polymerization of carbamate-functional acrylic monomers or by transcarbamylation of a hydroxy functional acrylic with an alkyl carbamate. Carbamate-functional polyesters, prepared by transcarbamylation of a hydroxy-functional polyester, are described in JP 51/4124.
Polyurethane resins are also widely used in curable compositions such as coating compositions. These resins offer many beneficial properties, such as good durability, good dispersibility in aqueous systems through incorporation of appropriate tonic or nontonic stabilizing groups, impact resistance, good adhesion, and other physical properties such as stress release. One area of concern with polyurethane resins for curable compositions has been the incorporation into the resin of sufficient levels of functional groups to achieve the desired cure performance. Hydroxyl groups are commonly used as functional groups in curable compositions, but polyurethane resins with pendant hydroxyl groups are difficult to prepare since any pendant hydroxyl groups would be consumed by reaction with isocyanate during formation of the polyurethane. Hydroxyl functional groups are usually incorporated onto polyurethane resins by the use of polyol capping agents like trimethylol propane resulting in terminal OH groups, but no pendant OH groups. Such resins provide only limited crosslink density upon cure. The crosslink density may be increased somewhat by using branched polyurethanes, which are prepared by the incorporation of trifunctional or higher functional polyols in the polyurethane reaction mixture. However, the degree of branching is often limited due to gelation. Low crosslink density must often be compensated for by using higher molecular weight resins that more closely resemble thermoplastic compositions than thermoset compositions.
Carbamate-functional polyurethanes are described in U.S. Pat. No. 5,373,069. This reference describes the preparation of polyurethanes having carbamate terminal groups by a variety of schemes, such as capping an isocyanate-terminated polyurethane with a hydroxyalkyl carbamate. The reference also discloses the preparation of polyurethanes having pendant carbamate groups by first preparing a polyurethane having pendant acid groups by known techniques, and then transesterifying the acid groups with a hydroxyalkyl carbamate. This approach involves a synthetic route that can be difficult. Moreover, it results in consumption of acid groups on the polyurethane, which are not then available for aqueous stabilization of the resin.
Accordingly, the present invention is directed toward a new method of preparing polyurethane polymers or oligomers having pendant carbamate groups. It is also directed to a polyurethane resin having both pendant carbamate groups and acid groups.